1. Field of the Invention
The present invention relates to a process for producing a polythiol oligomer with disulfide linkages in its backbone, which is useful as a raw material for optical polymer materials and devices. The process of the present invention affords a polythiol oligomer with a high refractive index and a high Abbe's number (or a low dispersive power). Therefore, the polythiol oligomer is suitable as a raw material to produce an optical polymer with good optical properties such as a high refractive index and a high Abbe's number (or a low dispersive power). The invention further relates to an optical polymer made from said polythiol oligomer and useful for the production of optical materials and devices. Polymers made from the above oligomers find wide application in, for example, optical lenses, eyeglass lenses, contact lenses, intraocular lenses, prisms, optical filters, optical fibers, optical disk substrates, etc.
2. Description of the Related Art
Plastics have recently come into common use in optical devices such as lenses because of their advantages over glass, in, for example, weight, crack resistance, and dyeability. Examples of optical plastics now in practical use include poly(diethylene glycol bisallyl carbonate), polymethyl methacrylate, and polycarbonate.
Glass and plastics used as optical materials generally decrease in Abbe's number as the refractive index increases, and vice versa. This means that it is generally very difficult to produce a plastic optical material having both a high Abbe's number and a high refractive index.
This difficulty has been overcome to some extent by a new plastic lens disclosed in Japanese Patent Laid-open No. 236386/1991. The reference relates to a polythiourethane lens obtained by reaction between a polythiol and a polyisocyanate. The disclosed polythiol is derived from 2,5-dimercaptomethyl-1,4-dithiane (referred to as DMMD hereinafter). Since DMMD has a high refractive index of 1.646 and a high Abbe's number of 35.2, the polythiourethane lens obtained from the polymerization of this monomer also has a high refractive index and a high Abbe's number. Despite these developments, there is still a strong demand for the development of new plastic lenses having higher refractive indexes and higher Abbe's numbers.
To meet this demand, Japanese Patent Laid-open Nos. 118263/1995 and 118390/1995 propose a process for producing another polythiourethane lens. The disclosed process includes oxidizing DMMD with air in the presence of a catalyst (such as methylsulfoxide and ferric chloride), thereby giving a mixture of DMMD oligomers, and reacting this mixture with polyisocyanate.
Unfortunately, the oligomers obtained by this process fluctuate in refractive index from 1.665 to 1.680 and in Abbe's number from 34.3 to 35.0, when reaction conditions such as temperature and humidity are slightly changed. Thus, the properties of the oligomer are unstable and an oligomer mixture having fixed, predictable properties is difficult to obtain. Consequently, it is difficult to form an optical polymer or material having a fixed refractive index and fixed Abbe's number. Therefore, the disclosed mixture of DMMD oligomers is of limited use for optical materials.
U.S. Pat. No. 2,237,625 discloses a process for producing a mixture of disulfide, trisulfide, and tetrasulfide by reaction between alkylmercaptan (monothiol) and sulfur in equimolar amounts in the presence of amine catalyst. B. D. Vinejard reported the same reaction as above catalyzed by n-butylamine, in Journal of Organic Chemistry, No. 32, p. 3833, 1967. However, these disclosed reactions are neither intended to synthesize polythiol oligomers from a polythiol with the functionality of two or above, nor to be used to produce optical polymer raw materials.